The present invention relates generally to press rolls for use in paper making machines and, more particularly, to press rolls useful as substitutes for granite rolls in paper making machines.
Stone rolls, often used in the center roll position of paper machine press sections, are usually formed of granite since a granite surface has generally acceptable web release properties and, additionally, is relatively resistant to the abrasive effects of a doctor.
However, granite press rolls have drawbacks resulting from other properties of granite. Granite is a non-homogeneous, brittle material having internal flaws inherent in its structure. Further, the properties of granite vary with heating and cooling cycles. This, in conjunction with the anisotropic properties of granite, can cause a number of problems which must be taken into account both in the manufacture and use of the rolls. For example, special grinding techniques at elevated temperatures have been used to improve the quality of granite rolls.
The fact that internal flaws exist in granite has limited the linear loads to which granite rolls can be subjected thereby limiting the extent to which a web can be dewatered in the paper machine press section. Although sophisticated designs, such as the use of tensioning flanges attached at the ends of a steel shaft passing through the thick cylindrical granite mantle for eliminating the possibility of tensile loads acting on the mantle under any loading conditions, have reduced the susceptibility of granite rolls to catastrophic failure, occasional cracking has occurred with consequent expensive down time required for repairs or replacement of the roll. Another disadvantage of granite rolls is their great weight which necessitates extensive frame constructions and which increase the tendency for the paper machine to vibrate during operation.
Attempts have been made for these reasons to provide synthetic substitutes for granite rolls. For example, rolls have been provided with coverings formed of a polymer, such as hard rubber or polyurethane, to which a rock powder, such as quartz sand, has been added. However, such synthetic rolls have the drawback that the paper web has a tendency to adhere excessively to the roll surface. Moreover, such rolls have poor strength properties and cannot be used at elevated temperatures, such as above 80.degree. C.